1. Field of the Invention
This invention relates to an evaporative fuel-purging control system for an internal combustion engine having an evaporative emission control system.
2. Prior Art
Conventionally, evaporative emission control systems have been widely used in internal combustion engines, which operate to prevent evaporative fuel (fuel vapor) from being emitted from a fuel tank into the atmosphere, by temporarily storing evaporative fuel from the fuel tank in a canister, and purging the same into the intake system of the engine. Purging of evaporative fuel into the intake system causes instantaneous enriching of an air-fuel mixture supplied to the engine. If the purged evaporative fuel amount is small, the air-fuel ratio of the mixture will then be promptly returned to a desired value, with almost no fluctuation.
However, if the purged evaporative fuel amount is large, the air-fuel ratio of the mixture fluctuates. For example, a large amount of fuel vapor can be produced in the fuel tank immediately after refueling or fill-up. In order to prevent fluctuations in the air-fuel ratio due to purging of evaporative fuel (fuel vapor) on such an occasion, there has been proposed (e.g., by Japanese Provisional Patent Publication (Kokai) No. 63-111277) a purging gas flow rate control system which reduces the purging amount of a mixture of evaporative fuel and air from the start of the engine immediately after refueling or fill-up until the speed of the vehicle in which the engine is installed reaches a predetermined value. The system also reduces the purging amount of the mixture after the vehicle speed has reached the predetermined value and until the accumulated time period over which the vehicle speed exceeds the predetermined value reaches a predetermined value.
Further, air-fuel ratio control systems are also known, which first effect purging of evaporative fuel in such a small amount as to cause almost no fluctuation of the air-fuel ratio, then detect an amount of variation of an air-fuel ratio correction coefficient applied to the feedback control of the air-fuel ratio, which is caused by the purging, next which forecast from the detected variation amount a value of the air-fuel ratio correction coefficient which should be assumed when the purged evaporative fuel amount is large, and thereafter apply the forecast value as the air-fuel ratio correction coefficient in the feedback control when the actual purged evaporative fuel amount becomes large. This reduces the fuel amount supplied to the engine, whereby fluctuations in the air-fuel ratio can be suppressed even when the purged amount is large (e.g. Japanese Provisional Patent Publication (Kokai) No. 62-131962).
However, the above conventional system is liable to fail in accurately controlling of the air-fuel ratio since the actual purged amount (the actual purged amount of the mixture of evaporative fuel and air) is not detected in controlling the flow rate of the purged mixture. More specifically, the amount of evaporative fuel produced by refueling and hence the resulting concentration of evaporative fuel in the mixture after refueling depend on an amount of fuel remaining in the fuel tank just before refueling, so that the amount of purged evaporative fuel after refueling varies. According to this conventional system, therefore, if the purging amount of the mixture is set to a relatively large value in expectation of the concentration of evaporative fuel in the mixture after refueling being relatively small, fluctuations can inevitably occur in the air-fuel ratio when a mixture with a high concentration of evaporative fuel is supplied by purging into the intake system. On the other hand, if the purging amount is set to a relatively small value in expectation of the concentration of evaporative fuel in the mixture after refueling being relatively large, the occurrence of fluctuations in the air-fuel ratio can be avoided, but the evaporative emission control cannot be performed to an adequate extent, if a mixture with a low concentration of evaporative fuel is then supplied by purging into the intake system.
Further, in the latter conventional system, the actual purged amount is not directly detected for the control of the air-fuel ratio, but the actual purged amount is estimated from the variation in the air-fuel ratio correction coefficient caused by the small purging amount, and at the same time, a variation amount in the air-fuel ratio to be caused by a large purging amount is forecast from the variation amount in the air-fuel ratio caused by the small purging amount. Therefore, the variation in the coefficient cannot be forecast accurately, which prevents accurate control of the air-fuel ratio from being carried out when purging of the evaporative fuel is effected.
Thus, both of the conventional systems can undergo fluctuations in the air-fuel ratio, resulting in degraded exhaust emission characteristics and fluctuations in engine output torque.